1. Field of the Invention
Zinc sulfide is a durable material which is intrinsically transparent to relatively long electromagnetic wavelengths in the far-infrared region. These properties contribute to its use in applications which require infrared transmission capability such as in infrared detectors and missile domes. Such zinc sulfide articles are typically produced by chemical vapor deposition (CVD) or hot pressing techniques. These techniques result in forms which are generally opaque and not functionally transparent in the visible or near-infrared regions of the electromagnetic spectrum. Hot-isostatic pressing (HIP) has been found to sufficiently improve the transparency of zinc sulfide articles in the visible and near-ultraviolet regions that these forms can be used in applications requiring multi-spectral capability, such as in tank and aircraft windows. However, difficulties in the final shaping/machining of these windows have compromised the capability of providing them in the precisely contoured shapes required for some applications, such as for some optical components. These difficulties have been particularly pronounced with attempts to fabricate relatively large articles. This invention relates to improvements in the CVD-Hipping process which have facilitated the consistent fabrication of precisely shaped, low stress, water-clear zinc sulfide articles.
2. Description of Related Art
Chemical vapor deposition has been used to produce zinc sulfide in bulk form or in the form of a layered coating on a substrate. Typically such production involves the reaction of H.sub.2 S with vaporized zinc in proximity to either a substrate or a mandrel box on which the zinc sulfide deposits, such as is described in U.S. Pat. No. 5,686,195. The zinc sulfide deposits produced by this CVD method generally exhibit many inclusions and poor transmission in the visible and near-infrared regions of the electromagnetic spectrum. As described in U.S. Pat. No. 4,944,900, the transmission properties in these regions can be substantially improved by hot isostatic pressing (HIP) of the form produced by CVD.
A moderately clear ZnS has been commercially produced by a two-step process. First, elemental zinc vapors are reacted with hydrogen sulfide at a H.sub.2 S/Zn molar ratio of 1, a mandrel (substrate) temperature of 735.degree. C. and an absolute pressure of 35 torr, in a CVD reactor. Zinc sulfide is deposited on the mandrel until a deposit of the desired thickness is produced. The deposit is separated from the mandrel to provide a zinc sulfide form which is then HIP treated for up to 100 hours at 900-1000.degree. C. and pressures of 15,000-30,000 psi. A recent application, Ser. No. 09/018,969 filed Feb. 5, 1998, describes an improvement of this process which reduced the number of visible inclusions present in the deposited form and provided a corresponding improved clarity of images viewed through articles prepared from these forms. The improved clarity of the product, referred to as water-clear zinc sulfide, has resulted in an expanded range of applications including some which require physical shaping/grinding to very precise final contours. Customers, however, have complained of difficulty accurately machining windows from 15-20 inch forms of water-clear zinc sulfide to a specification which required a figure of less than one tenth of a wave RMS (root mean square) at a wavelength of 632.8 nm. Exceeding this specification resulted in product windows which demonstrated unacceptable distortion of the transmitted images.
It was suspected the machining difficulties were related to a high level of stress birefringence, in the range of 80-240 nm/cm., which was measured in large water-clear zinc sulfide forms. These high stress birefringence values were generally measured near the edges of the forms, while relatively low values were measured near the forms' centers. Since considerable machining of the as deposited silicon carbide form, to reduce both its thickness and its edges, is required to result in the desired window shape, it was suspected such machining was responsible for the high stress birefringence values. Careful control of the machining and etching process provided a limited reduction of the total stress birefringence measured in forms; however, the stress birefringence per unit of window thickness was not appreciably reduced. A further attempt to reduce the stress values involved repeating the HIP treatment and controlling the cool down to less than 31.degree. C. per hour. The second HIP treatment reduced the stress birefringence at the edges from a value in excess of 100 nm/cm to about 70 nm/cm. While this second HIP treatment demonstrated a reduction in the stress value, it was not sufficient to provide the required machinability. Providing a series of HIP treatments to achieve a satisfactory level of stress birefringence was not considered an economically viable approach, since each HIP treatment requires heat soaking the form at high temperature and high pressure for up to 100 hours in a fairly large, high temperature rated, pressure vessel.
Accordingly, there is a need for forms of machinable, low stress, water-clear zinc sulfide, particularly relatively large forms, i.e. those having thicknesses greater than 1/2 inch and/or a maximum face dimension (length and/or width) greater than 10 inches. There is also a corresponding need for a process capable of consistently producing such low stress, water-clear zinc sulfide forms.